Steel pipe pole base and reinforcing method thereof

ABSTRACT

The present invention: provides a steel pipe pole base and a method for reinforcing the steel pipe pole base that do not allow the strength to lower in the vicinity of the weld toe of a rib even in the event of applying repeated bending moment; and is characterized by forming peening processed portions  20  by ultrasonic vibration at the weld toes  16  of tabular ribs  12  welded to the base of a steel pipe pole  10  in the form of a T-joint or the weld toes of inverted-U shaped ribs  13  or inverted-V shaped ribs  14,  the ribs being bent at the upper end portions, welded to the base of a steel pipe pole  10  in the form of a T-joint. Preferable treatment conditions are 20 to 50 μm in amplitude and 10 to 50 kHz in frequency. In particular, the present invention makes it possible to remarkably improve the fatigue property because stress concentration is relaxed by the synergetic effect of bending the upper end portions  16  of the ribs  13  and applying a peening treatment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a steel pipe pole base and areinforcing method of the steel pipe pole base for, for example, fixinga steel pipe pole such as a street light support pole, a road signsupport pole and the like to a skeleton such as road and the like.

2. Description of the Related Art

As a steel pipe pole base for fixing a steel pipe pole such as a streetlight support pole, a road sign support pole and the like to a skeletonmade of concrete and the like, a structure constructed by welding a baseplate 11 to the lower end portion of a steel pipe pole 10 andreinforcing the joint between the steel pipe pole 10 and the base plate11 with a plurality of ribs 12, as shown in FIG. 10, is generallyemployed. Each of the ribs is a tabular triangular rib the upper endportion of which is cut obliquely and is welded to the steel pipe pole10 in the form of a T-joint. Then, the steel pipe pole 10 is verticallysupported by fixing the base plate 11 to the skeleton with anchor bolts15.

However, in a conventional steel pipe pole base as described above,there has been a danger that, when a bending moment is imposed on asteel pipe pole 10 due to wind, vibration or the like, a large stressconcentrates on the steel pipe pole 10 near the weld toes 16 of ribs 12and, as a consequence, the strength at the portions deteriorates due tothe repeated stress. Another problem has been that structural defectsare likely to occur in the boxing welded portions at the upper endportions of the ribs 12 as a result of the combined effect of theresidual tensile stress and the material degradation of theheat-affected zones caused by welding heat and to cause the proof stressand the fatigue property to deteriorate.

Those problems are common to joint structures in which reinforcing ribsare welded to structural members in the form of a T-joint and, in viewof this, the Japanese Society of Steel Construction points out, in“Guidelines for Fatigue Design of Steel Structures and ItsInterpretation”, that a joint in which a gusset is welded by filletwelding adversely affects the proof stress and fatigue property of asteel member and therefore attention has to be paid to the design ofstructures.

SUMMARY OF THE INVENTION

The present invention has been established for solving theaforementioned conventional problems and providing a steel pipe polebase and a reinforcing method of the steel pipe pole base, those makingit possible to: suppress the deterioration of strength in the vicinityof the weld toe of a rib even when a repeated bending moment is imposedon the steel pipe pole; and prevent the deterioration of the proofstress and fatigue property of a boxing welded portion at the upper endportion of the rib. The gist of the present invention is as follows:

(1) A steel pipe pole base reinforced with ribs welded to said steelpipe pole base in the form of a T-joint, characterized by formingpeening processed portions at weld toes by ultrasonic vibration.

(2) A steel pipe pole base according to the item (1), characterized bysaid ribs being tabular ribs.

(3) A steel pipe pole base according to the item (1), characterized bysaid ribs being inverted-U or inverted-V shaped ribs bent at the upperend portions.

(4) A method for reinforcing a steel pipe pole base according to any oneof the items (1) to (3), characterized by applying peening treatment toweld toes by ultrasonic vibration after said tabular ribs, inverted-Ushaped ribs or inverted-V shaped ribs are welded to said steel pipe polebase in the form of a T-joint.

(5) A method for reinforcing a steel pipe pole base according to theitem (4), characterized by applying peening treatment to said weld toesby ultrasonic vibration while a load is imposed on said steel pipe polebase so as to impose a tensile stress in the direction of the steel pipeaxis on the base material in the region subjected to said peeningtreatment.

(6) A method for reinforcing a steel pipe pole base according to theitem (4) or (5), characterized by applying said peening treatment byultrasonic vibration under the conditions of 20 to 50 μm in amplitudeand several tens of kHz in frequency.

As mentioned above, in the present invention, peening treatment isapplied by ultrasonic vibration to the weld toes of inverted-U orinverted-V shaped ribs formed by bending the upper end portions oftabular ribs welded to a steel pipe pole base in the form of a T-joint.The method employed for the peening treatment is a method wherein acylindrical tool is ultrasonicalls vibrated in the axis direction, thetip of the vibrating cylindrical tool is applied to the surface of anobjective metal and, by so doing, the surface is made concave. Thismethod makes it possible to strengthen a steel pipe pole base byimposing a high level energy on a metal surface, thus producing plasticdeformation, relaxing stress concentration, and imposing residualcompressive stress on a weld toe.

Further, in the present invention, by employing inverted-U or inverted-Vshaped ribs as mentioned above, the upper end portions of the ribs areliberated from the principal stress direction of a steel pipe pole to adirection perpendicular to the principal stress direction and therigidity of the rib upper end portions is lowered. As a result of this,it is possible to considerably relax stress concentration produced atweld toes, when bending stress is imposed on a steel pipe pole, and alsothe residual tensile stress caused by welding heat.

Furthermore, in the present invention, a peening treatment is applied tothe weld toes of ribs by ultrasonic vibration. The method employed forthe peening treatment is a method wherein a cylindrical tool isultrasonically vibrated in the axis direction, the tip of the vibratingcylindrical tool is applied to the surface of an objective metal, and byso doing the surface is concaved. In consequence, a high level energy isimposed on the weld toes, plastic deformation is produced, and residualcompressive stress is imposed. For this reason, the weld toes that havebeen the weak points of a steel pipe pole base are further strengthenedand therefore it becomes possible to suppress the deterioration ofstrength at the weld toes of ribs and prevent the deterioration of theproof stress and the fatigue property, of boxing welded portions at theupper end portions of the ribs, even when a repeated bending moment isimposed on a steel pipe pole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the first embodiment according tothe present invention.

FIG. 2 is views explaining a portion subjected to peening treatment byultrasonic vibration in the first embodiment shown in FIG. 1; FIG. 2(a)is a side view, 2(b) a front view and 2(c) a perspective view.

FIG. 3 is a front view showing the second embodiment according to thepresent invention.

FIG. 4 is a perspective view showing the third embodiment according tothe present invention.

FIG. 5 is a stress concentration profile obtained by subjecting a steelpipe pole base to which an inverted-U shaped rib is attached to FEManalysis.

FIG. 6 is a stress concentration profile obtained by subjecting a steelpipe pole base having a conventional construction to FEM analysis.

FIG. 7 is S-N curves showing the results of fatigue strength tests inthe case of Example 1.

FIG. 8 is S-N curves showing the results of fatigue strength tests inthe case of Example 2.

FIG. 9 is a side view of an ultrasonic impact device.

FIG. 10 is a perspective view showing a conventional example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Firstly, a steel pipe pole base reinforced with tabular ribs accordingto the present invention is explained hereunder.

In FIG. 1, reference numeral 10 denotes a steel pipe pole used as astreet light support pole, a road sign support pole or the like, 11 abase plate welded to the lower end portion of the steel pipe pole 10, 12a plurality of ribs welded in the form of a T-joint for thereinforcement between the steel pipe pole 10 and the base plate 11. Eachof the ribs 12 is tabular and cut obliquely at the upper end portion andforms a triangular rib. Then, in the case of a road pole, the steel pipepole 10 is vertically supported by fixing the base plate 11 to askeleton made of concrete by using anchor bolts 15. The aboveconfiguration is the same as a conventional one.

In the present invention, a peening processed portion 20 subjected toultrasonic vibration is formed at the weld toe 16 of each tabular rib 12as shown in FIG. 2. The peening treatment by ultrasonic vibration isapplied by the method as shown in FIG. 9 wherein the tip of acylindrical tool 22 of an ultrasonic impact device 21 is applied to thesurface of an objective metal, is ultrasonicalls vibrated in the axisdirection, and by so doing makes the surface concave.

The tip of the cylindrical tool 22 generally has a round section and apreferable diameter thereof is about 1 to 6 mm. The reason is that, whena diameter is less than 1 mm, the strength is insufficient and enoughimpact cannot be imposed, and, in contrast, when a diameter exceeds 6mm, the mass is too large and therefore ultrasonic vibration is hardlygenerated.

A preferable frequency of the cylindrical tool 22 is in the range from10 to 50 kHz and a preferable value of amplitude thereof is in the rangefrom 20 to 50 μm. The reason for regulating the frequency as above isthat a large impact energy can be imposed efficiently on a steelmaterial in that frequency range. When a value of amplitude is less than20 μm, a sufficient impact cannot be imposed. On the other hand, when avalue of amplitude exceeds 50 μm, the plastic deformation of a steelmaterial undesirably increases excessively in some cases.

A metal surface processed under the aforementioned conditions undergoesplastic deformation by a high level energy, is made concave to a depthof about 0.1 to 0.5 mm, and a tensile stress can be introduced up to thedepth of 10 mm or more from the surface. Further, the metallographicstructure changes largely up to the depth of about 100 μm from thesurface, a texture layer called a white layer is formed, and goodcorrosion resistance, good wear resistance and the reduction of frictionresistance can be obtained.

In the present invention, such a peening processed portion 20 subjectedto ultrasonic vibration as mentioned above is formed at the weld toe 16of each tabular rib 12 as shown in FIG. 2. It is preferable to form thepeening processed portion 20 at a portion from the upper end portion ofthe rib 12 to at least about 10 mm downward. As a result, the stressconcentration at the weld toe 16 is relaxed, a tensile stress is imposedon the weld toe 16, and the fatigue strength improves remarkably.Further, though the weld toe 16 of a rib 12 is a portion wherestructural detects are likely to occur by the combined effect ofresidual tensile stress and the degraded heat-affected zone caused bywelding heat as mentioned above, even structural defects such as finecracks can be remedied by changing the microstructure by applyingpeening treatment by ultrasonic vibration.

Next, a steel pipe pole base reinforced with inverted-U or inverted-Vshaped ribs bent at the upper end portions according to the presentinvention is explained hereunder.

In FIG. 3, reference numeral 10 denotes a steel pipe pole used as astreet light support pole, a road sigh support pole or the like, 11 abase plate welded to the lower end portion of the steel pipe pole 10, 13a plurality of inverted-U shaped ribs welded in the form of a T-jointfor the reinforcement between the steel pipe pole 10 and the base plate11. Here, the inverted-U shaped ribs 13 may be replaced with inverted-Vshaped ribs 14 as shown in FIG. 4. The base plate 11 is fixed to askeleton such as a road by using anchor bolts 15 and the steel pipe pole10 is supported vertically.

The steel pipe pole 10 undergoes the principal stress in the verticaldirection and the ribs 13 or 14 also stretch as a whole in the principalstress direction of the steel pipe pole 10. However, the upper portionof each of the ribs 13 is bent gradually in the shape of a circular arcand the upper end portion 16 of each of the ribs 13 that forms a weldtoe is bent to the extent of forming a right angle with the direction ofthe principal stress of the steel pipe pole 10.

In this way, by gradually bending the upper end portion 16 of each rib13 in such a direction as to be liberated from the principal stressdirection of a steel pipe pole 10, the upper end portion 16 of each rib13 can be formed into a structure having a low rigidity. As a result,stress concentration at the upper end portion 16 of each rib 13 isrelaxed, residual weld thermal stress at a weld is also relaxed greatly,and the proof stress and the fatigue property as a welded structure areimproved considerably.

In order for these effects to be achieved sufficiently, it is preferablethat the radius of curvature at the upper end portion 16 of each rib 13is set at not less than three times the thickness of the rib 13. If theradius of curvature is less than the above value, the material qualitytends to deteriorate when a rib 13 is bent and also the effect oflowering the rigidity is lessened.

Here, FIGS. 5 and 6 show stress concentration profiles obtained bysubjecting a steel pipe pole base shown in FIG. 3 and a conventionalsteel pipe pole base shown in FIG. 10 to FEM analysis. Each of thoseprofiles shows by contour lines the distribution of stress generated inthe vicinity of a rib 13 or 14 when a horizontal load is equally appliedto the upper end portion of a steel pipe pole 10 and the unit of thenumerals in each profile is MPa. It can be understood, from a comparisonbetween FIGS. 5 and 6, the maximum value of stress concentration isreduced up to half that of a conventional structure by bending the upperend portion 16 of a rib 13.

Note that the arrows in the upper right direction shown in FIGS. 7 and 8mean that the loading was terminated since no change in the specimenswas observed at the time. Further, the expression “n=2”, means that thenumber of the specimens is two. In this regard, the weld quality of thepartial specimens used in the tests is of a very high grade andtherefore the fatigue life may possibly be shortened to some extent atthe ordinary industrial production level.

Moreover, in the present invention, the weld toe of each of the bentribs 13 is further subjected to peening treatment by ultrasonicvibration. A peening processed portion 20 is defined by the regionextending at the central angle α on both sides of the center line of therib 13 as shown extendedly in FIG. 3 and a preferable angle α isgenerally in the range from 30 to 60 degrees. The angle α in FIG. 3 isabout 45 degrees.

In this way, in the steel pipe pole base according to the method of thepresent invention, as the weld toe of each of the inverted-U shaped ribs13 or the inverted-V shaped ribs 14 bent at the upper end portions 16 issubjected to peening treatment by ultrasonic vibration, the effects ofthose constituents are combined together. As a result, it is possible toconsiderably relaxation stress concentration generated in the vicinityof the weld toe of each of the ribs 13 or 14 when a bending moment isimposed on a steel pipe pole 10 due to wind, vibration or the like, andto conspicuously improve the fatigue strength at the portion as shown inthe data of the examples described later.

Further, it is generally acceptable to apply peening treatment byapplying the tip of a cylindrical tool 22 of an ultrasonic impact device21 to the weld toe of each of ribs 12, 13 or 14 welded to the base of asteel pipe pole 10. However, it is also acceptable to apply peeningtreatment by ultrasonic vibration while a load (a bending load forexample) is imposed on a steel pipe pole base so that a tensile stressin the steel pipe axis direction is applied to the base material in thetreatment region. In this way, by applying peening treatment to andimposing a compressive stress on a weld toe while a tensile stress isimposed by externally given force, it becomes possible to make a farlarger compressive stress remain at the weld toe 16 when the externallygiven force is removed. Consequently, a far more excellent reinforcingeffect can be obtained.

Though peening treatment is applied by ultrasonic vibration to only theweld toe of each of the ribs 12, 13 or 14 in the above explanations,needless to say, it is also acceptable to apply peening treatment toother welded portions. However, it is estimated that the application ofpeening treatment to the lower portions or the like of the ribs 12, 13or 14 is not practically beneficial because the portions do not directlyaffect the fatigue strength of the steel pipe pole base.

EXAMPLE 1

Fatigue strength tests were carried out by imposing repeated tensilestress on partial specimens around steel pipe ribs having theconstruction shown in FIG. 1. The material used for both the steel pipesand the ribs was SM 490. The fatigue property of the conventionalconstruction having as-welded ribs as shown in FIG. 10 corresponded tothe E to D grades of the design life curves in the “Railway BridgeDesign Guideline” as shown by the black round marks in FIG. 7. On thecontrary, the fatigue property of the invention construction whereinpeening treatment was applied to weld toes by ultrasonic vibrationmarkedly improved up to the grade B or higher of the design life curvesas shown by the white round marks in the figure. Here, the amplitude ofthe tool tip was 40 μm and the frequency thereof was 30 kHz.

Further, when peening treatment was applied by ultrasonic vibrationwhile a load for applying tensile stress was imposed on a steel pipepole base, the fatigue property improved up to the grade A of the designlife curves as shown by the black triangular mark. Moreover, even whenpeening treatment was applied to the weld toe where fatigue cracks weregenerated by ultrasonic vibration, the fatigue property improved up tothe grade A of the design life curves as shown by the white triangularmarks. The data show that peening treatment by ultrasonic vibration hasthe function of remedying fatigue cracks.

EXAMPLE 2

Fatigue strength tests were carried out by using a test device andimposing repeated tensile stress on the steel pipe pole bases accordingto the present invention as shown in FIGS. 3 and 4. The material usedfor both the steel pipes and the ribs was SM 490. In comparison, fatiguestrength tests were carried out by imposing repeated tensile stress onpartial specimens around steel pipe base ribs having the constructionshown in FIG. 10. As a result, the fatigue property of the conventionalconstruction shown in FIG. 10 corresponded to the E to D grades of thedesign life curves in the “Railway Bridge Design Guideline” as shown bythe black round marks in FIG. 8. On the contrary, the fatigue propertyof the invention construction markedly improved up to the grade A of thedesign life curves as shown by the white round marks in the figure.Here, the amplitude of the tool tip was 40 μm and the frequency thereofwas 30 kHz.

Further, when peening treatment was applied by ultrasonic vibrationwhile a load for applying tensile stress was imposed on a steel pipepole base, the fatigue property improved up to the A grade or higher ofthe design life curves as shown by the white triangular marks.

1. A steel pipe pole base reinforced with ribs welded to said steel pipepole base in the form of a T-joint, characterized by forming peeningprocessed portions at weld toes by ultrasonic vibration.
 2. A steel pipepole base according to claim 1, characterized by said ribs being tabularribs.
 3. A steel pipe pole base according to claim 1, characterized bysaid ribs being inverted-U or inverted-V shaped ribs bent at the upperend portions.
 4. A method for reinforcing a steel pipe pole baseaccording to any one of claims 1 to 3, characterized by applying peeningtreatment to weld toes by ultrasonic vibration after said tabular ribs,inverted-U shaped ribs or inverted-V shaped ribs are welded to saidsteel pipe pole base in the form of a T-joint.
 5. A method forreinforcing a steel pipe pole base according to claim 4, characterizedby applying peening treatment to said weld toes by ultrasonic vibrationwhile a load is imposed on said steel pipe pole base so as to impose atensile stress in the direction of the steel pipe axis on the basematerial in the region subjected to said peening treatment.
 6. A methodfor reinforcing a steel pipe pole base according to claim 4,characterized by applying said peening treatment by ultrasonic vibrationunder the conditions of 20 to 50 μm in amplitude and several tens of kHzin frequency.